1. Field of the invention
The present invention relates to a boring apparatus for use in drilling deep wells, i.e., wells for recovering petroleum deposits or for tapping underground water sources or for use in civil construction work. More particularly, the present invention relates to a boring apparatus in which a cutter holder with an attached inner cutter is moved by water under pressure through a drill pipe toward the lower end of the drill pipe located in the ground, wherein the cutter holder is locked by a locking device on the lower end of the drill pipe, and wherein the drill pipe is rotated and pushed into the ground while rotating the cutter holder so as to enable an inner cutter and outer cutters mounted on the cutter holder to drill an underground hole having a diameter larger than the outside diameter of the drill pipe.
2. Description of the Prior Art
Boring apparatus of the type described have reamer cutters for enlarging an underground hole as drilled by a cutting bit to a size larger than the outside diameter of a drill pipe so that the drill pipe can be advanced through the hole as the hole is drilled. U.S. Pat. No. 3,894,590 discloses a boring apparatus having reamer cutters disposed closer to a ground surface than the bit located centrally in the underground hole being formed function to enlarge the hole.
The present inventor has found that the peripheral speed of a boring apparatus is larger at a position radially away from a center of rotation than at such a center of rotation, and boring can be performed more efficiently than heretofore by cutting the outer circumferential portion of a cylindrical hole with outer cutters in advance of a bit which cuts off a central portion of the cylindrical hole. For such boring operation, it is necessary that the outer cutters be positioned further underground than the bit on the cutter holder.
The outer cutters mounted on the cutter holder should be accommodatable within the drill pipe so that the cutter holder can be freely moved through the drill pipe. With the boring apparatus disclosed in U.S. Pat. No. 3,894,590, however, the outer cutters are located closer to the ground surface that the bit, and hence the cutter holder is required to have housing regions for accommodating therein the outer cutters. Since such housing regions are defined in a portion of a shank to which the bit is secured, this means that this portion of the shank has a reduced sectional area and is mechanically weak. To provide the shank with a required degree of mechanical strength, the size of the outer cutter used is limited, and the cutting efficiency of the outer cutters cannot be increased.
According to the boring apparatus described in U.S. Pat. No. 3,894,590, water is introduced into the drill pipe and supplied through the cutter holder toward the bit and outer cutters to cool the bit and outer cutters, and ground rocks cut by the bit and outer cutters are discharged with the water up along the outer surface of the drill pipe toward the ground surface. For effectively supplying water to the cutting edges of the bit and outer cutters, the prior boring apparatus has an annular packing extending around the cutter holder and pressed against an annular step defined on an inner surface of the drill pipe so that water will be supplied from within the drill pipe into and through the cutter holder to the bit, and no water will pass between the cutter holder and the drill pipe toward the lower end of the drill pipe. When the cutter holder is to be lifted through the drill pipe toward the ground surface, a mass of water filled in the drill pipe acts as a large load on the annular packing, resulting in difficulty in smoothly lifting the cutter holder through the drill pipe.